Ultrafiltration (UF) is a technology utilized for water purification which separates dissolved molecules on the basis of size by passing a solution through a fine filter. The ultrafilter is generally a tough, thin, selectively permeable membrane that retains most macromolecules and/or particles above a certain size including colloids, microorganisms and pyrogens. Thus, UF provides a retained fraction (retentate) that is rich in large molecules and/or particles and a filtrate that contains few, if any, of these molecules and/or particles.
Current state-of-the-art ultrafiltration technology utilizes ceramic micro-filters or perforated hollow fibers. Problems with these filtering systems include membrane fouling and membrane cost, which are responsible for the high cost and low efficiency treatment of fluids such as water, including bilge water (water that collects and stagnates in the bilge of a ship) and produced water (water produced during the production of hydrocarbons, which is considered a contaminant of oil and gas). The symptom of fouling is an unavoidable consequence of gradual blockage of the permeability of the membrane during filtration. The fouling rate is a function of many variables, such as the surface characteristics of the membrane, the surface-to-volume ratio of the membrane, the flow rate, the permeant concentration, filtration temperature, and the character of the feed and retentate streams.
Today, commercially available polymeric membrane systems can be categorized mainly into three different types: (1) hollow fiber filters, (2) tubular filters, and (3) spiral wound filters. The hollow fiber filter consists of bundled hollow fibers with an internal diameter of about 1 mm. The advantage of this filter system is its low cost, physical strength and light weight. The disadvantages of this system are its tendency to clog, fiber breakage, and it is more accessible to chemical and biological attack. The tubular filter consists of individual tubes with internal diameters of about 1 cm. It is not subject to clogging but is subject to chemical and biological attack. The low packing density of tubular filters also leads to low filtration efficiency. The spiral wound filter consists of rolled up sheet membranes with a supporting spacer. It has the advantage of lower cost and is light weight, but it is also subject to chemical and biological attack.
Research is ongoing for suitable polymers for membrane applications. Many polymer systems, such as cellulose acetate, polyacrylonitrile (PAN), polyethersulfone (PES), polyamides, and polyvinylidenefluoride (PVDF) have been used in conventional ultra-filtration and nano-filtration membranes. Very often, homopolymer systems alone are not sufficient to achieve good filtration properties. Chemical modifications, such as copolymerization, polymer grafting and crosslinking, as well as physical modifications such as polymer blending, have been used to improve the hydrophilicity and biological functions for the filtration of water such as bilge water. Currently; several research groups have been exploring new material systems for membrane applications. These systems include amphiphilic graft copolymers based on PVDF, poly[1-(trimethylsilyl)-1-propyne] (poly(TMSP)), crosslinked lyotropic liquid crystal assembly, ion selected nanostructured supramolecular membranes, hollow helical molecules, and block copolymers containing hydrophilic poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) and hydrophobic poly(1,1′-dihydroperfluorooctyl methacrylate) (PFOMA) or poly(1,1,2,2-tetrahydroperfluorooctyl acrylate) (PTAH) block.
The high fouling rate of low-cost filters (e.g., polymeric hollow fibers and spiral wound membranes) requires the frequent replacement of these filters, resulting in a correspondingly high operational cost. High-cost ceramic filters, although capable of overcoming some of the disadvantages of low-cost polymeric filters, do not fully address these challenges.
Improved filters for use in ultrafiltration systems, which are lower in cost and have lower fouling at higher rates of flux, are thus desirable.